1. Technical Field
The present disclosure relates to stabilized lithium powder, a negative electrode using the stabilized lithium powder, and a lithium ion secondary battery using the stabilized lithium powder.
2. Related Art
An electrochemical device typified by a lithium ion secondary battery, whose positive electrode employs a lithium-containing transition metal oxide typified by lithium cobaltate and whose negative electrode employs a carbon material capable of being doped and de-doped with lithium, has a characteristic feature of high energy density. Due to this feature, such an electrochemical device is important as a power source for a portable electronic appliance typified by a mobile phone. Along with the rapid spread of the portable electronic appliances, the demand for the lithium ion secondary battery has been increasing.
A number of eco-friendly vehicles including hybrid vehicles have been developed. As a power source to be mounted on the hybrid vehicle, a lithium ion secondary battery with high energy density has attracted attention.
The capacity of the lithium ion secondary battery mainly depends on an electrode active material. As a negative electrode active material, graphite is typically used. To meet the above demand, however, it is necessary to use a negative electrode active material with higher capacity. In view of this, metal silicon (Si) with much higher theoretical capacity (4210 mAh/g) than that of graphite (372 mAh/g) has attracted attention.
One of the known means for improving the performance of the lithium ion secondary battery is a pre-doping technique. According to this technique, mainly the negative electrode of the lithium ion power storage device is doped with lithium ions in advance, so that the irreversible capacity of the electrode in the lithium ion power storage device can be suppressed.
For example, Japanese Patent No. 4126157 discloses a vertical pre-doping method in which a holed foil with a penetration hole is used as a current collector. In the vertical pre-doping method, in addition to a positive electrode and a negative electrode, a third electrode is used for supplying lithium ions to the positive electrode and the negative electrode.
A fabrication process with the vertical pre-doping method is more complicated than a normal fabrication process for the lithium ion power storage device, and therefore requires more time and cost for fabricating the device. In another technique, a lithium foil is used for the entire positive electrode mixture layer and negative electrode mixture layer. However, lithium is so soft that it is very difficult to attach the lithium foil evenly on the electrode mixture layer. Moreover, it is difficult to handle the lithium foil in this work, so that the mass productivity may be affected.
For solving those problems, a method of using lithium powder has been suggested in Japanese Patent No. 4126157. According to this method, a dispersion solution obtained by dispersing the lithium powder in a solution is applied on a negative electrode active material, thereby conducting pre-doping. Moreover, in the disclosure, the stabilized film is formed on the surface of the powder particle included in the lithium powder used in the pre-doping for the purpose of increasing the stability (stabilized lithium powder). However, the reliability of the battery using the stabilized lithium powder in a long-term cycle has not been clarified sufficiently. Further improvement has been desired.